This invention is in the field of solar energy collecting systems of the so-called focusing collector type and is particularly concerned with support and thermal protection of getters fixed to the metal tube radiation absorbers of such solar energy systems.
The terms getter, getters and getter material will be used interchangeably hereinafter in the specification and claims.
For reasons such as the rapid exploitation of non-renewable energy resources, the desire for environmentally friendly energy sources and the advantages to locally provide energy to remote communities and factories, there is a continued interest in solar energy systems and improvements thereto. Continuous research increases efficiency of solar systems which gradually become more cost effective and render them more favorable.
A particular type of solar collector system is the xe2x80x9cfocusing collectorxe2x80x9d type which enhances the effect of solar energy by focusing it onto a smaller area using mirrored surfaces or lenses. In this system, a reflector, which is typically parabolic, receives and reflects (focuses) incoming solar radiation onto a metal tube radiation absorber (TRA)xe2x80x94in the form of a tube; the TRA being concentrically surrounded by a treated glass enclosure tube to limit the loss of heat. To further increase the efficiency by increasing the collection of solar radiation, the collector system typically includes means to track the sun.
To minimize the loss of heat through convection and conduction and to improve the solar radiation collection efficiency, the space between the tube radiation absorber (TRA) and the glass enclosure tube is evacuated to very low pressure.
The TRA is made of metal with a coating having a high solar radiation absorption coefficient to maximize the energy transfer imparted by the solar radiation reflecting off the reflector. A heat transfer medium, which for practical considerations is typically a liquid such as oil, flows within the TRA.
At the high temperatures inherent of solar collectors, the heat transfer medium releases some amount of hydrogen. The hydrogen released, being such a small atom, may permeate through the wall of the TRA and enter the space between the TRA and the glass enclosure tube. This situation is very undesirable as it reduces the vacuum in that space thereby allowing heat loss from the TRA via convection and conduction. Accordingly, it is critical to remove this hydrogen to preserve the efficiency of the focusing solar collector.
One method for removing hydrogen from the space is by use of a getter located therein, which is a material that may comprise a variety of metal alloys having a high surface area and affinity for adsorbing hydrogen. Other active gaseous impurities such as CO, CO2, N2 and H2O, which may find their way into the space, can also be thereby adsorbed.
Data published on the equilibrium of commercial hydrogen getters indicates that hydrogen getters may have an enormously increased capacity and gas adsorption rate with a relatively small reduction in temperature thereby greatly affecting the cost of a solar collector system. In order to maintain the capacity and rate of gas adsorption by the getter, the getter must be supported in the space between the TRA and the glass enclosure tube and be maintained at relatively low temperature. Thus, the getters should be as isolated as possible from heat transfer such as from conduction and radiation.
Radiation may come from either of two sources, one being radiation emitted off the TRA. Despite the fact that the TRA has a coating to limit emission, there is still a non-negligible amount of radiation that the TRA emits. The second radiation source potentially heating the getters is so-called missed solar radiation. This radiation can result from situations wherein some solar radiation is not solely reflectedly focused onto the TRA, but rather misses it. This is typically due to reflector mis-alignment or imperfect TRA mounting position. Some of this xe2x80x9cmissed solar radiationxe2x80x9d may directly or indirectly contact the getters on their support and thereby heat them.
U.S. Pat. No. 4,306,543, to Doevenspeck et al, discloses a solar collector comprising a getter located within an evacuated transparent envelope defined by an outer glass tube and an inner metal tube. A rigid frame construction, extending between the glass and metal tubes, supports the getter and also supports a heater. The aim of the design is to provide a heating element in the evacuated envelope to crack hydrocarbons that may be present. However, the design does not provide protection for the getters from heating.
U.S. Pat. No. 4,455,998, to Kroontje et al, discloses a solar collector comprising a reversibly heatable hydrogen getter located within an evacuated transparent envelope defined by a glass tube. A reservoir formed in the glass tube supports the getter, which is covered by gauze. However, the reservoir is only described in regards to a plate-shaped absorber solar collector. Also, the reservoir requires a specially configured glass tube, the reservoirxe2x80x94being part of the glass tubexe2x80x94and this reservoir is not insulated from the heat absorbed by the glass tube. Further, the described assembly is designed to alternatively heat or not heat the getter in order to help control the temperature of heat transfer medium in the collector; not to maintain the getter at a low temperature.
U.S. Pat. No. 4,508,104, to Takeuchi et al, also discloses a solar collector comprising a getter located within an evacuated transparent envelope. A retaining device composed of leaf springs functions to fix the location of the inner tube of the collector with respect to the outer tube. The retaining device also supports a grommet-shaped casing of a xe2x80x9cgetter metal.xe2x80x9d The portion of the retaining device supporting the getter metal is only described as C-shaped and does not appear to protect the getter from solar radiation.
Accordingly, it is an object of the present invention to provide an assembly, for a solar energy collector system of the solar radiation focusing type, for supporting getters and maintaining them at relatively low temperature.
According to the present invention there is provided an assembly for supporting getters in an evacuated space between a tube radiation absorber (TRA) and the glass tube of a solar collector system, comprising a bridge for providing a gap between the getters and the tube radiation absorber wherein the bridge is designed and assembled of components to have minimal surface contact therebetween, thereby limiting conductive heat transfer.
According to the present invention there is provided such an assembly comprising a radiation protection arrangement for limiting solar radiation from reflecting and/or emitting onto the getters or onto a component (i.e. the bridge) in direct or indirect contact with the getters.
The present invention provides a getter support assembly for supporting getters in a solar collector system of the focusing collector type comprising a tube radiation absorber (TRA) and a glass enclosure tube, defining therebetween an annular space, and a solar radiation focusing reflector, said getter support assembly comprising, a bridge formed with an elongated trough having a getter support portion, the bridge further comprising feet fixedly attached to the tube radiation absorber supporting said trough so that the trough is spaced apart from said tube radiation absorber; the trough further comprising a radiation reflecting surface facing the TRA for blocking radiation emitted from the TRA and missed solar radiation reflected from the reflector.
According to one embodiment of the present invention, the bridge further comprises a component in addition to the trough, namely a radiation shield, to block and reflect radiation in order to minimize the temperature of the getters.
The bridge further comprises slots providing axial freedom of movement to allow for thermal expansion of the TRA and the bridge.